As a pile is driven, the operator of the pile driver must be careful to control the force of the blows applied to the pile so as not to exceed the elastic limit of the pile material in order to minimize costly tip damage. Those who are familiar with pile driving are well acquainted with the condition known in the art as "overdriving" and the ultimate damage that results therefrom. To minimize such damage, drive caps are fitted over the head of the pile to evenly transmit the hammer blows to the pile, while at the same time maintaining the head of the pile in alignment with the hammer by guiding the head parallel to the leads frame and retaining the pile in a straight predetermined path.
Though this helps to mitigate some of the damage, the burden in most cases falls largely upon the experience of the operator to determine the driving force required. For a given set of conditions, tests may be made to help the judgment of the operator and further, strain gauges may be used to determine the force of the blow and the dynamic forces within the pile. Strain gauges, however, must be cemented or otherwise attached to the beam, and it is a relatively time consuming and costly process to mount them properly. The strain gauge is also a very fragile device and its reliability under the repetitive dynamic shock loading to which the pile is subjected is, to say the least, questionable.
After a pile is driven to the proper depth, there remains a need for measurement of the static bearing load which the pile will support, which is usually done by loading a test pile with weight until it moves. This is called a dead load bearing test and is also a time consuming and expensive process. By accurate measurement of the dynamic forces below which the pile will not move, the bearing capacity may be reasonably estimated thus saving considerable time and expenses.
The invention disclosed herein, as well as the method by which it is achieved, provides a novel means for utilizing magnetostrictive principles for accurately and quickly determining the driving force applied to the end of the pile, the visual indication of which can be displayed directly to the operator of the pile driver for proper adjustment and accurate control of the driving force. The principles of this invention can be further used to provide a means for determining the equivalent static bearing capacity of the pile after it is driven to a position where it should have attained its bearing capacity. Magnetostriction can be described as the deformation of a material causing the generation of an electric current which influences a magnetic field, or vice versa, i.e., deforming a ferrous material under the influence of the magnetic field and inducing a current which accordingly changes the magnetic field. Further, the change in the magnetic field is proportional to the deformation within the elastic limit, thus such deformation may be determined by the magnitude of the change. Normally, this effect is small and noticeable under most conditions, however, the tremendous driving force used in driving a pile gives rise to usable signals which can be sensed with instruments of normal sensitivity and be used to determine the force with which the pile was struck.